Such a circuit configuration conventionally has a control loop with an operational amplifier whose inverting input is connected with the output terminal via a capacitor, whose noninverting input is connected with a reference current source and whose output is connected with the control input of a transistor. The output terminal is connected via a resistor with a first supply voltage terminal, which normally has a positive potential, and via the transistor with a second supply voltage terminal, which usually has ground potentials
The capacitor supplies the inverting input of the operational amplifier with a capacitor current EQU Ic=C.cndot.dU/dt, (1)
where C is the capacitance of the capacitor, U the voltage drop across the resistor and t the time. At constant voltage, i.e., outside the edges of a pulsed signal, the capacitor current is Ic=0, while it is nonzero during the presence of pulse edges. The operational amplifier adjusts the capacitor current to the value of the reference current, which corresponds to a very definite steepness of the voltage drop across the resistor.
One performs slew rate control for example to reduce the high-frequency electromagnetic interference connected with steep pulse edges.
The output terminal of the control circuit can be connected for example to a bus line, e.g., one of the double lines of a CAN bus system as used nowadays in motor vehicles. The abbreviation CAN stands for controlled area network.
In such bus systems, for example, it can happen that the bus line connected to the output terminal of the control circuit has at some place a short circuit to the positive supply voltage terminal. In this case the resistor between the output terminal of the control circuit and said positive supply voltage terminal is short-circuited. Since such a short circuit leads to a constant potential value at the output terminal of the control circuit, the capacitor current becomes zero and the control circuit attempts to drive the transistor to maximum current output in order to bring the capacitor current back to the current strength of the reference current source.
The problems entailed by such a short circuit, namely high current consumption and high power dissipation, can be remedied by providing a short-circuit protecting circuit that causes a switch-off of the control circuit when the transistor current has exceeded a certain protective threshold.
Although such a protecting circuit offers protection from lengthy short circuits, serious problems still remain.
The load acting at the output terminal of the control circuit is virtually always inductive, at least because a line connected to said output terminal has a line inductance. A high short-circuit current before the time of protective switch-off results in accordingly high magnetic energy collecting in the load inductance and leading at the time of protective switch-off to inductive voltage pulses that can assume relatively high voltage levels.
Such voltage pulses result in relatively high electromagnetic interference, on the one hand, and involve the danger of signal decoders in the form of comparators connected to the line misinterpreting such inductive voltage pulses as signal or data pulses, resulting in a falsification of data transferred via the line, on the other hand.